RECENT DEVELOPMENTS IN PHYSICS. LP F 5012 191- 3 9004 01511217 7 ,,I5ithWlorne pierce Queen's University at Kingston RECENT DEVELOPMENTS IN PHYHICS. THE NATURE OF LIGHT EMISSION. IT would be difficult to find a man on the street who has not heard of wireless telegraphy and electric waves. Few men, however, realize that their own eyes are constantly receiving wireless messages, that light waves are electrical in nature. When electric waves are sufficiently small the eye becomes their detector, and we have the sensation of light. The long waves sent out by the wireless operator on board ship are but the big brothers of the light waves emitted by a luminous body. It is only within the last fifty years, however, that this fact has been brought to light. In 1865, J. Clerk Maxwell as a result of brilliant mathematical analysis was led to enun- ciate his "Electromagnetic Theory of Light." Thirteen years later the theory received striking experimental confirmation. Hertz, experimenting on electric waves in his laboratory, showed that these waves could be reflected, refracted, polar- ized and in other respects conducted themselves exactly like light waves. The brilliant work of Hertz left little doubt of the truth of Maxwell's theory, and since his time investigation has but confirmed it. At the present time the theory is universally accepted. The physicist of the twentieth century is directing his at- tention to what is going on at the source of light. Just what is the nature of light emission? Of what nature is the vi- brating body sending out the minute electric waves ? Are its vibrations subject to mathematical analysis? These and other questions he is seeking to answer. In this note a short outline of the present position is given. It will be noted first of all that if we accept the theory that light waves are electrical in nature, we must have an electrical source. This has been supplied for us by work in other fields of Physics. In another article in this number of the Quarterly, it is pointed out that atoms of all substances contain definite numbers of electrons, negatively charged par- ticles of very small mass. It is in the vibrations of these electrons that we find our source of light waves. Sometimes 498 QUEEN'S QUARTERLY. the atoms of substances are not allowed to pursue the even tenour of their way, — a little common salt is put in a gas flame, or an electric discharge is sent through a tube contain- ing a rarefied gas — a commotion takes place among the elec- trons, they are set vibrating and short electric waves are sent out. These, falling on our eyes, give us the sensation of light. We must refer next to the question of the analysis of the light emitted by any luminous body. If we pass ordinary sunlight through a glass prism (a spectroscope) we find an emergent "rain-bow" of light with colors running from red to violet. If, however, we examine the light emitted when an electric spark passes between, say, two pieces of iron, we now find a spectrum consisting of a great many coloured lines, each distinct from the other. The same result is obtained when we analyze with our prism the light emitted by a gas made luminous by an electric discharge. It has been shown, fur- ther, that these spectral lines occur in series, that a definite mathematical relation obtains with reference to the positions they occupy in the spectrum. In other words, if we know the position of one line, from a general mathematical expres- sion we can calculate the position of the, others. The explan- ation of this realm of law concerning spectral lines is evident if we remember that the prism simply analyzes into simple components the very complex vibrations within the atom. Each spectral line corresponds to a simple vibration, a great number of which make up the actual resultant vibration at the source. We have referred to work on spectral series because it is closely related to one of the outstanding problems concerning the minds of physicists at the present day. This problem is just the converse to the above. Is it possible to take the atom with its electrons and mathematically predict what spectral lines should be emitted ? Can we analyze mathematically the vibrations sent out by a disturbed electron and show that the components bear the same ratio to each other as that obtain- ing among spectral lines? This is the problem now demand- ing solution at the hands of mathematical phisicists.. Already considerable advance has been made. Within the last year papers have appeared in several of the leading physical jour- nals, in which it has been shown that for simple atoms, at any RECENT DEVELOPMENTS IN PHYHICS. 499 rate, the problem is soluble. Undoubtedly the near future will show marked advances in this field of enquiry. The problem of the nature of light emission is being at- tacked also by experimental physicists, the most prominent of whom is Prof. Wood of Johns Hopkins University. For some years he has been investigating the fluorescent properties of vapours. The general reader will recall that certain sub- stances emit light when stimulated by light from some exter- nal source. They are said to fluoresce, or, — if the light con- tinues to be emitted on removal of the outside source — to phos- phoresce. It is possible, therefore, to cause bodies to emit light under the stimulus of an external "exciting" source. The general reason for this is evident. The exciting light waves, being electrical in nature, set into vibration the electrons within the atoms of the substance on which they fall, thus causing a light emission from the latter. Now it will at once be clear that for fluorescence to take place, there must be some sort of "tuning" between the frequency of the vibra- tions falling on an atom and the natural frequency of the vibrations which itself would emit. In other words we cannot expect to obtain fluorescence for all vibrations which may fall on a body, and experiment shows that this is the case. As both the exciting light and the fluorescent light may be analyzed with a spectroscope, we have a means of examining any relation which may exist between them. We are thus supplied with another method of attacking the problem of the nature of light emission. In this field Prof. Wood has carried out already several brilliant researches. His work has been confined to a study of the fluorescent properties of vapours, substances which, on account of their comparative simplicity of structure, render the problem much less complex. He has found tliat even the slightest variation in the nature of the exciting vibrations, produces marked differences in the fluorescent light. By controlling the vibrations of the source within very narrow limits, he hopes to throw considerable light on our knowledge of the internal vibrations of the atom. The brilliant work he has accomplished already is a voucher that much may be expected from him in the future. J. K. R. 500 QUEEN'S QUARTERLY. THE QUANTUM THEORY. One of the greatest triumphs of modern Physics is the firm establishment of the atomicity of matter ; that matter is built up of definite sized units. Ostwald, the great German chemist, and his band of followers were for many years the chief opponents of this theory, but in face of the accumulated evidence collected by the physical chemists, Ostwald was led to accept the atomic theory. Now, we are able to count the molecular units in a given mass of matter, measure their size and follow their movements. We have learned also, that instead of being a simple thing, as originally supposed, the atom has electrical constituents. Then again, it has been discovered that the electrical part of the atom of matter has itself an atomic structure. The value of the charge of an elementary electrical unit has been definitely measured in many ways, and has been found invariable. The latest atomic theory deals with energy and is called the quantum theory. Of course, we all recognize electricity as a manifestation of energy and now scientists place matter in the same category ; so that the quantum theory is really the old atomic theory generalized to apply to all kinds of energy. It was first proposed by Planck about ten years ago in order to account for the way in which heat energy leaves a heated body. Many attempts had previously been made to develop an equation expressing the relation between the temperature of a body, and the rate at which heat left it, or in other words its rate of cooling. Newton, Du Long and Petit, and various others had worked out empirical f ormulae,which. however, held only for small ranges of temperature. Wien and Rayleigh, mathematical physicists, had each, on theoretical grounds, de- veloped a formula, which was found incorrect when tested by experiment. The only assumption they had made was that heat energy is continuously emitted by a hot body. Planck attacked the same problem and assumed from the beginning, that the emission of energy from the heated body is not con- tinuous, as they supposed, but is sent off at intervals in small units or so called "quanta." From this he deduced a relation which, contrary to all the other ones, agreed well with the experimental facts. Since then Planck's idea has been applied to all kinds of interchanges of energy. Nerst, a chemist, has RECENT DEVELOPMENTS IN PHYH1CS. 501 shown that the quantum theory will account for the way in which the specific heats of bodies change as they are cooled to very low or heated to very high temperatures, and Einstein and J. J. Thomson have formulated a quantum theory of light which is radically different from the older wave theory. The Einstein-Thomson theory is that light, instead of being given out continuously by an illuminated body and propogated in the form of waves through the ether, is sent off from the body by means of energy spots, each spot containing an integral num- ber of energy units or quanta. Accordingly, a beam of light is not a continuous wave structure but consists of energy spots distributed at random through it, all of which are moving with the same velocity. It is rather remarkable that this theory resembles Newton's corpuscular theory which was abandoned in favour of the wave theory. In view of recent work, it is necessary to adopt an atomic or quantum theory with regard to X-rays as opposed to a wave theory. An X-ray pulse produced by the stopping of a cathode particle hurls out an electron from a molecule. It is found that the amount of energy required to throw an electron from a molecule is the same as that required to stop the electron which produced the X-ray. The X-ray, then, must simply transfer energy from one electron to another. To do this, it is apparent that the X-ray cannot be a spherical wave pulse which spreads from the point where it is made. Rather, it must be a form of localised energy which travels directly to the place where it ejects an electron. Only in this way could the same amount of energy be transferred from the place where the X-ray is made to the place where it is used up in driving an electron from a molecule of matter. The X-rays then must have an atomic structure. When we come to consider the late experiments of Laue, Friedrich, and Knipping, and those of Bragg and others, we find that X-rays are very similar to ordinary light rays in their properties. It is most probable then, that ordinary light is built up in the same way as X-rays, and that Einstein and Thomson's quantum theory of light is correct,, Whatever the ultimate fate of the 'quantum theory, it seems probable that it will do its part in pointing the way to 5 o2 QUEEN'S QUARTERLY. new investigations. We have tried to show how it has co- related experimental data, which no other theory could do. Any theory is strong in proportion to its power to do these things and the quantum theory seems to have won its place among the other theories of natural phenomena, when judged by such a standard. V. E. P.